I extended the water inlet of my AC 70 since my 38g is 19" height. So I extended the inlet so it extends to the buttom of the tank. Now my question is would the extra depth decreases the filter turn over rate (GPH)? I think I noticed it does, but not sure.

The height shouldn't affect the flow rate through the filter a significant amount. The biggest factor is how high the filter has to lift the water above the tank's water level. How did you go about extending the intake? If the intake is more constricted, it may decrease the flow rate.